Black-list jumbo1.js from continuous stepdown suite on 3.2 branch

jumbo1.js starts the balancer in the middle of execution. The 3.2 branch doesn't support automatically retrying that operation, because it's done in the shell rather than through a command running on mongos. Since the test cannot be fixed using features available in 3.2, we should blacklist the test in the continuous stepdown suite on that branch.